Transparent conductors are widely used in the flat-panel display industry to form electrodes that are used to electrically switch the light-emitting or light-transmitting properties of a display pixel, for example in liquid crystal or organic light-emitting diode displays. Transparent conductive electrodes are also used in touch screens in conjunction with displays. In such applications, the transparency and conductivity of the transparent electrodes are important attributes. In general, it is desired that transparent conductors have a high transparency (for example, greater than 90% in the visible spectrum) and a high conductivity (for example, less than 10 ohms/square).
Typical prior-art conductive electrode materials include indium tin oxide (ITO) and very thin layers of metal, for example silver or aluminum or metal alloys including silver or aluminum. These materials are coated, for example, by costly vacuum deposition methods such as sputtering or vapor deposition, and patterned on display or touch screen substrates, such as glass. Patterning is typically done by traditional multi-step lithographic processes. However, the current-carrying capacity of such electrodes is limited, thereby limiting the amount of power that can be supplied to the pixel elements. Moreover, the substrate materials are limited by the deposition process (e.g. sputtering). Thicker layers of metal oxides or metals increase conductivity but reduce the transparency of the electrodes.
Various methods of improving the conductivity of transparent conductors are taught in the prior art. For example, issued U.S. Pat. No. 6,812,637 entitled “OLED Display with Auxiliary Electrode” by Cok, describes an auxiliary electrode to improve the conductivity of the transparent electrode and enhance the current distribution. Such auxiliary electrodes are typically provided in areas that do not block light emission, e.g., as part of a black-matrix structure.
It is also known in the prior art to form conductive traces using nano-particles comprising, for example silver. The synthesis of such metallic nano-crystals is known. For example, issued U.S. Pat. No. 6,645,444 entitled “Metal nano-crystals and synthesis thereof” describes a process for forming metal nano-crystals optionally doped or alloyed with other metals. U.S. Patent Application Publication No. 2006/0057502 entitled “Method of forming a conductive wiring pattern by laser irradiation and a conductive wiring pattern” describes fine wirings made by drying a coated metal dispersion colloid into a metal-suspension film on a substrate, pattern-wise irradiating the metal-suspension film with a laser beam to aggregate metal nano-particles into larger conductive grains, removing non-irradiated metal nano-particles, and forming metallic wiring patterns from the conductive grains. However, such wires are not transparent and thus the number and size of the wires limits the substrate transparency as the overall conductivity of the wires increases.
Touch screens with transparent electrodes are widely used with electronic displays, especially for mobile electronic devices. Such devices typically include a touch screen mounted over an electronic display that displays interactive information. Touch screens mounted over a display device are largely transparent so that a user can view displayed information through the touch screen and readily locate a point on the touch screen to touch and thereby indicate the information relevant to the touch. By physically touching, or nearly touching, the touch screen in a location associated with particular information, a user can indicate an interest, selection, or desired manipulation of the associated particular information. The touch screen detects the touch and then electronically interacts with a processor to indicate the touch and touch location. The processor can then associate the touch and touch location with displayed information to execute a programmed task associated with the information. For example, graphic elements in a computer-driven graphic user interface are selected or manipulated with a touch screen mounted on a display that displays the graphic user interface.
Touch screens use a variety of technologies, including resistive, inductive, capacitive, acoustic, piezoelectric, and optical technologies. Such technologies and their application in combination with displays to provide interactive control of a processor and software program are well known in the art. Capacitive touch screens are of at least two different types: self-capacitive and mutual capacitive. Self-capacitive touch screens can employ an array of transparent electrodes, each of which in combination with a touching device (e.g. a finger or conductive stylus) forms a temporary capacitor whose capacitance can be detected. Mutual-capacitive touch screens can employ an array of transparent electrode pairs that form capacitors whose capacitance is affected by a conductive touching device. In either case, each capacitor in the array can be tested to detect a touch and the physical location of the touch-detecting electrode in the touch screen corresponds to the location of the touch. For example, U.S. Pat. No. 7,663,607 discloses a multipoint touch screen having a transparent capacitive sensing medium configured to detect multiple touches or near touches that occur at the same time and at distinct locations in the plane of the touch panel and to produce distinct signals representative of the location of the touches on the plane of the touch panel for each of the multiple touches. The disclosure teaches both self- and mutual-capacitance touch screens.
Referring to FIG. 10, a prior-art touch screen and display system 100 includes a display 110 with a corresponding touch screen 120 mounted with the display 110 so that information displayed on the display 110 is viewed through the touch screen 120. Graphic elements displayed on the display 110 can be selected, indicated, or manipulated by touching a corresponding location on the touch screen 120. The touch screen 120 includes a first transparent substrate 122 with first transparent electrodes 130 formed in the x-dimension on the first transparent substrate 122 and a second transparent substrate 126 with second electrodes 132 formed in the y-dimension facing the x-dimension first transparent electrodes 130 on the second transparent substrate 126. A dielectric layer 124 is located between the first and second transparent substrate 122, 126 and first and second transparent electrodes 130, 132. Referring also to the top view of FIG. 11, in this example first pad areas 128 in the first transparent electrodes 130 are located adjacent to second pad areas 129 in the second transparent electrodes 132. (The first and second pad areas 128, 129 are separated into different parallel planes by the dielectric layer 124.) The first and second transparent electrodes 130, 132 have a variable width and extend in orthogonal directions (for example as shown in U.S. Patent Publication Nos. 2011/0289771 and 2011/0099805, which are hereby incorporated by reference. When a voltage is applied across the first and second transparent electrodes 130, 132, electric fields are formed between the first pad areas 128 of the x-dimension first transparent electrodes 130 and the second pad areas 129 of the y-dimension second electrodes 132.
Another prior-art disclosure of a touch screen with variable-width electrodes and sensing cells is found in U.S. Patent Publication No. 2011/0248953. Conductive dummy patterns are located between adjacent sensing cells at different heights above a transparent substrate and the conductive dummy patterns partially overlap the sensing cells.
A display controller 142 controls the display 110 in coordination with a touch screen controller 140. The touch screen controller 140 is connected through electrical buss connections 136, 134 and controls the touch screen 120. The touch screen controller 140 detects touches on the touch screen 120 by sequentially electrically energizing and testing the x-dimension first and y-dimension second transparent electrodes 130, 132.
Referring to FIG. 12, in another prior-art embodiment, rectangular first and second transparent electrodes 130, 132 are arranged orthogonally on first transparent substrate 122, 126 with intervening dielectric layer 124, forming touch screen 120 which, in combination with the display 110 forms a touch screen and display system 100. First and second pad areas 128, 129 are formed where the first and second transparent electrodes 130, 132 overlap. The touch screen 120 and display 110 are controlled by touch screen and display controllers 140, 142 respectively, through electrical busses 136 and 134.
Since touch screens are largely transparent, any electrically conductive materials located in the transparent portion of the touch screen either employ transparent conductive materials (for example, transparent conductive metal oxides such as indium tin oxide) or employ conductive elements that are too small to be readily resolved by the eye of a touch screen user. Transparent conductive metal oxides are well known in the display and touch screen industry and have a number of disadvantages, including inadequate transparency and conductivity, high manufacturing costs due to vacuum processes and a tendency to crack under mechanical or environmental stress. Further, the high demand for indium has significantly increased the price of this raw material. Thus, touch screens including very fine patterns of conductive elements, such as metal wires or conductive traces can provide a useful alternative. For example, U.S. Patent Publication No. 2011/0007011 teaches a capacitive touch screen with a mesh electrode, as does U.S. Patent Publication No. 2010/0026664.
Referring to FIG. 13, a prior-art x- or y-dimension variable-width transparent electrode 130, 132 includes a micro-pattern 156 of micro-wires 150 arranged in a rectangular grid. The micro-wires 150 are multiple very thin metal conductive traces or wires formed on the first and second transparent substrates 122, 126 to form the x- and y-dimension transparent electrodes 130, 132. The micro-wires 150 are so thin that they are not readily visible to a human observer, for example 1 to 10 microns wide. The micro-wires 150 are typically opaque and spaced apart, for example by 50 to 500 microns, so that the first and second transparent electrodes 130, 132 appear to be transparent and the micro-wires 150 are not distinguished by an observer.
U.S. Patent Publication No. 2009/0219257 discloses a touch screen sensor that includes a visible light transparent substrate and an electrically conductive micro-pattern disposed on or in the visible light transparent substrate. The micro-pattern includes a first region micro-pattern with a first sheet resistance value and a second region micro-pattern with a second sheet resistance different from the first sheet resistance value. As disclosed, the second region sheet resistance is lower than the first and includes micro-breaks in the conductive micro-pattern.
Mutually-capacitive touch screens typically include arrays of capacitors whose capacitance is repeatedly tested to detect a touch. In order to detect touches rapidly, highly conductive electrodes are useful. In order to readily view displayed information on a display at a display location through a touch screen, it is useful to have a highly transparent touch screen. There is a need, therefore, for an improved method and apparatus for providing electrodes with increased conductivity and transparency in a mutually capacitive touch screen device.